Butter: Some Technology and Chemistry
The history about the definition and standard for butter in the United States can be traced back to legislation Congress enacted in 1886. Many changes have occurred since that date, leading to the present statutory standard that Congress enacted into law on March 4, 1923:
Butter shall be understood to mean the food product usually known as butter which is made exclusively from milk or cream, or both, with or without common salt, and with or without additional coloring matter, and containing not less than 80% by weight of milk fat, all tolerances having been allowed for.
Butter production in the United States during 1999 totaled 1.28 billion pounds, 9.2 percent above 1998. California accounted for 26.9 percent of the production, followed by Wisconsin with 22.8 percent and Washington with 9.3 percent. However, per capita consumption of butter has continued to decline. Undoubtedly, many reasons--nutritional, economic, and aesthetic--can be cited for this decrease.
A 1986 survey conducted by the United Dairy Industry Association among approximately 4,000 U.S. consumers aged thirteen years and older revealed that most consumers prefer butter to margarine, claiming that they can taste the difference. Also, most of the respondents agreed that butter is a healthful food, despite media reports to the contrary. Almost 60% believed that butter has more nutritional value than margarine, while 61% believed that margarine is not as fattening as butter.
When groups advertize butter, the advertizing campaign stress the taste, aroma, freshness, and natural ingredients of butter and remind viewers hat butter is superior to its competition at enhancing the taste of food. No mention is made of the textural aspect of butter which is also an important attribute of the overall unique butter characteristics.
Butter is one of several dairy products that may be subjected to the inspection and grading services of the Dairy Division of the U.S. Department of Agriculture. Butter manufactures must request these services, qualify for them, and pay a fee based on the cost of providing the services. Participation in this program leads to improvement of quality, wholesomeness, manufacture, and distribution of butter.
The U.S. grade of butter is determined by first the flavor characteristics and then the characteristics of body, color, and salt content.
The following comment is made in the above reference about the body of butter.
Butter with a firm waxy body has an attractive appearance, has granules that are close knit, cuts clean when sliced, and has good spreadability. The trier sample from such butter will show this clean cut smooth, waxy appearance. The temperature of the butter at the time of grading is important in determining the true characteristics of body and should be between 45oF and 50oF.
USDA graders evaluate body in butter for its characteristics or defects. There are eight major defects considered in this evaluation: (1) crumbly (lacks cohesion); (2) gummy (sticky mouthfeel); (3) leaky (free moisture on the butter surface); (4) mealy or grainy (a grainy feel on the tongue similar to cornmeal); (5) ragged boring (unable to draw a smooth full trier of butter); (6) short (lacks plasticity and tends toward brittleness); (7) sticky (butter sticks to trier as a smear); and (8) weak (lacks firmness).
Most of the above defects in body can be associated with the spreadability characteristics of butter. USDA has made the following general statement about body characteristics:
Butterfat in butter is a mixture of various triglycerides of different melting points and appears as fat globule and free fat. In both forms, part of the fat is crystalline and part liquid. Some fats are solid at temperatures up to 100oF or even higher. Others are still liquid at temperatures far below the freezing point. Butter, at the temperature at which it is usually handled, is always a mixture of crystallized and liquid fat. The variations in the composition of milk fat thus have great influence upon the body and spreadability of butter. In the summer when milk fat contains more liquid or soft fat, butter tends to be weak and leaky. In the winter when the milk fat contains more solid fat, butter tends to be hard and brittle, resulting in unsatisfactory spreadability. The ratio between the crystalline and liquid fat particles depends upon the composition of the milk fat (varying with the season of the year), manufacturing methods, and the temperature of the butter. Close attention needs to be given to tempering the cream, temperature of churning, washing and working of the butter as the seasons of the year change. This is important in maintaining a uniform firm, waxy body possessing good spreadability.
Thus, many factors evidently play a role in determining the body and spreadabilty characteristics of butter. Unfortunately, when butter is removed from the refrigerator (40-45oF), the spreadability is limited. One must allow the butter to temper at room temperature to obtain optimum spreadability. Much research has been conducted which is directed toward improving the spreadability of butter. The remainder of this article will consider the various aspects of manufacturing butter with improved spreadability.
Manufacture of Butter
The theory of churning proposed by King is still considered plausible today:
The normal churning process is confined to a comparatively narrow temperature range with an optimum value for the ratio of crystalline to solid to liquid fat. When cream foams, fat globules come to the air--a serum interface of air bubbles. Liquid fat from the globules spreads at the interface along with material of fat globule membranes. The film of liquid fat cements globules into clumps. On repeated formation and destruction of foam bubbles, clumps grow to butter granules that contain modified serum (buttermilk) in the interstices between the fat globules. During working of the mass, some fat globules are crushed, and their contents are added to liquid fat. Also, moisture droplets are subdivided and air is entrapped.
Most butter in the U.S. today is manufactured in a continuous churn. However, some butter is still manufactured by the conventional batch method that was prevalent for many years before the introduction the continuous churn. In either case, the butter is manufactured under rigid sanitary requirements. The cream must be pasteurized before churning at a temperature equal to or greater than 74oC for a minimum of thirty minutes or else at a temperature equal to or greater than 85oC for a minimum of fifteen second; most cream is pasteurized by the latter method today.
A huge metal cylinder that turns around a horizontal axis is most often used for churning butter by the conventional batch method. As the churn rotates, the cream is agitated, and several thousand pounds (8,000) of butter can be manufactured in one churning.
The following steps are usually followed in batch churning: "(1) prepare the churn by cleaning and sanitizing, (2) pump cream of 30-33% fat at 9oC in summer or 13oC in winter into a churn, (3) add coloring, (4) rotate the churn until butter granules are formed (breaking point) and become the size of peas or popcorn, (5) drain buttermilk, (6) rinse buttermilk from interior surfaces of the churn with clean, cold water, (7) wash butter with sufficient water to bring total volume to that of original cream (water colder than butter firms, where as water warmer than butter softens), (8) drain wash water, (9) add salt, (10) work butter sufficiently to bring granules and water into compact mass, (11) sample and test for moisture, (12) add water if insufficient (below about 16%) or permit to escape from the churn if the test shows high moisture, (13) work butter until it has a firm, waxy body, (14) sample and test for moisture, salt and curd, and (15) remove butter from the churn."
Butter is then packaged in automated printer-wrapper machines, being cut into one-quarter or 1-lb prints (sticks), wrapped in foil or parchment, and then stored at -18o to -29oC.
Manufacture of butter by the continuous method did not become applicable until the late 1940s, although methods were introduced at the end of the 19th century. Continuous manufacture of butter is based on one of two principles: (1) accelerated churning and working and (2) reseparation with phase inversion.
In most butter-producing nations today, the first principle is employed in continuous butter manufacture, the Fritz method being the best-known process. In this process, which employs the same principles as conventional batch churning, crystallization of milk fat takes place in the cream, with phase inversion and concentration of milk fat occurring in the churn.
The steps in manufacture of butter by the Fritz process are as follows. Cream (40-42% milk fat) is fed into the double-cooled churning cylinder fitted with beaters driven by a variable-speed motor. Phase inversion takes place very rapidly, with the butter grains and buttermilk passing on to the separation section, where the butter is separated from the buttermilk. Butter grains are washed with recirculated buttermilk en route to the separation section (first working section). Butter working begins in this section while the butter is being conveyed by the screw conveyor to the squeeze-drying section.
In the squeeze-drying section, the butter passes through a conical channel and perforated plate to remove any remaining buttermilk. The butter grains continue to the second working section, which operates at the speed of the screw in the first working section to achieve optimum working of the butter. Upon reaching the injection section, a high-pressure injector may add salt. The vacuum working section is attached to a vacuum pump, allowing air to be removed from the butter to improve physical properties and keeping quality.
The final working stage consists of four small sections separated from each other by a perforated plate. The perforations are of varying diameters so that the butter receives satisfactory treatment as it is being worked. The first section contains a water injector for final adjustment of moisture content, which deviates by less that 0,1% if the cream characteristics remain constant.
The most modern continuous butter churn manufactured by Westfalia, Oelde, Germany (Type BUD) is also a Fritz system. Cream of 40% milk fat content is tempered and fed to the first churning cylinder, forming the basic butter granules. The prechurned mixture of granules and buttermilk then enters a second churning cylinder, which is a rotating drum (two-thirds screened). There, granules are compacted, and the buttermilk is discharged through the screen into a buttermilk a collecting vessel. This vessel contains a rotating screen basket that recovers residual butter granules.
Butter granules from the second churning cylinder then enter the texturizer, where remaining buttermilk is squeezed from the butter granules and necessary adjustment is made for moisture control and salt addition. The butter then passes to a second texturizer system via a vacuum chamber where gas and air can be drawn off. Thorough intermixing and texturizing are accomplished in the second texturizer, and the finished butter is then discharged from the churn.
Moisture control is the most serious problem in this process. The treatment of the cream (fat content and temperature crystallization conditions) and the churning process must be controlled for uniform moisture content. The most common moisture control unit is based on the dielectric properties, but infrared methods are also available. Most Fritz-type continuous churns have a capacity of 1,800-11,000 lb. of butter/hour. However, a computer-operated machine is now available with a capacity of 22,000 lb/hour.
Factors affecting Spreadability
Since milk fat is the major constituent of butter, it obviously plays a major role in determining its textural properties. The major fatty acids found in milk fat and their melting points are listed in Table 1. The fatty acid composition, distribution of fatty acids in the triglycerides, and the polymorphic forms of the fat crystals influence the melting properties of the milk fat. Melting takes place between 30 and 41oC.
Other factors also affect spreadability:
The cow’s diet and stage of lactation influence the fatty acid composition of milk fat. Thus, cyclic changes which can be monitored influence the melting properties of milk fat. In the northern U. S., butter tends to become firmer during the fall and winter because of dietary intake changes, i.e., an increased consumption of high-molecular-weight saturated fatty acids.
Studies have been conducted with the objective of altering the composition of milk fat by feeding. For example, introduction of soybean oil into the diet resulted in a higher oleic acid content and a butter with a lower melting point and reasonable spreadability at refrigeration temperature. However, because of economic factors and physiological effects on the cow, this procedure has gained only limited use.
Pretreatment of Cream
Much research has been conducted over the past forty years to improve the consistency of butter through temperature pretreatment of the cream before churning. Such treatment results in controlled crystallization of the milk fat. The Swedish or Alnarp: 6-21-12" method has gained wide acceptance in many countries.
In this process, after pasteurization, the fat in fat globules is in the liquid form, but the fat begins to crystallize when the cream is cooled to below 40o C. Quick cooling of the cream to a low temperature results in the rapid formation of many small fat crystals. However, the ratio of liquid to solid fat would be low and would result in a hard butter. Heating such cooled cream to a higher temperature melts the higher-melting -point triglycerides from the crystals. Recrystallization of the melted fat at a lower temperature results in a higher liquid/solids ratio, yielding a softer butter.
Treatment of Butter
Although treatment of cream before churning is considered the most feasible means of improving the spreadability of butter, attempts have been make to work butter after churning to improve it. One of the most interesting properties of butter is its tendency to soften during working. The greater part of softening occurs instantaneously, and the properties of the butter determine the amount of the breakdown rather than the intensity of working.
Ivarson showed that holding freshly churned butter at 5oC for a few hours, then working it in a compact mixer will result in improved spreadability at refrigeration temperature. However, the present practice of manufacturing butter continuously usually precludes the opportunity for such treatment since the butter is packaged almost directly after churning. The mixing process apparently results in deformation of the crystalline structure of the butter, yielding improved spreadability and slow recovery of firmness.
Other approaches have been taken to improve the spreadability of butter.
Whipping a gas into it can improve the spreadabililty of butter, nitrogen now being commonly used. While such butter possesses improved spreadability, it tends to be crumbly and does not possess as much butter aroma as unwhipped butter. Nevertheless, there may be an increase in consumption of whipped butter in the U.S.
As butter must contain no fat other than milk fat, manufacturing a blend is not possible and still call the resultant product butter. However, in the U.S. in recent years manufacturers have introduced margarines that are blends of vegetable fat and milk fat (60:40). These products possess improved spreadability but lack the fine flavor quality of butter.
The process of fractionation has given considerable interest to the possibility of improving the texture of butter. For example, short-path distillation has been used to obtain butter enriched in short- and medium-chain triglycerides. However, this process requires the use of high temperature, with the possibility of decomposition or polymerizaton occurring. Reducing the time of exposure by use of a wiped-film evaporator or centrifugal still reduces this possibility.
Boudreau and Arul have investigated the use of supercritical. For example, in one experiment, anhydrous milk fat was separated into tow fractions, the liquid one being enriched in short- and medium-chain triglycerides and fatty acids, and the solid fraction enriched in both saturated and unsaturated long-chain fatty acids. While this process provides high-quality milk-fat fractions, it is costly from both energy and capital cost standpoints.
Fractionation of milk fat by crystallization from the melt (dry fractionation) has been thoroughly studied. The liquid fraction becomes enriched in the short-chain triglyceride and both short-chain and unsaturated fatty acids. However, the efficiency of this process is low, and flavor compounds, pigment, vitamin A, and cholesterol are concentrated in the liquid fraction. There is one active industrial process, the Tiriaux system, which uses dry fractionation.
It is possible to fractionate milk fat by crystallization in organic solvents such as acetone. However, the loss of flavor compounds, pigment alteration, and presence of solvent residues in the milk-fat fractions has prevented industrial application.
A chemical approach to improving the texture of milk fat is the process of Interesterification, which alters the distribution of fatty acids. In this process, the triglyceride composition and the physical properties of the fat are changed, while the original total fatty acid composition is retained. While this process can improve the spreadability of butter, it has not yet been applied because it results in loss of the fine flavor of butter. Also, interesterification via enzyme catalysis is considered too costly presently to be feasible.
Measurement of Textural Properties
Many techniques have measured textural properties of butter, the penetrometer being the most widely reported. Sectility testing, in which a cutting wire is loaded until it cuts through a sample of butter at a constant rate, has also been studied. Compression testing using a basic rheometer to produce load/deformation curves, has yielded information on the viscosity and moduli of elasticity for butter. Extrusion of a plug of butter through an orifice has been used as an index of spreadability. Finally, attempts have been made to determine the spreadability of butter by using a blade that scrapes along the surface to simulate the spreading action.